Field effect transistors (FET) are used in many applications, particularly in power switches because of their robust ability to withstand current and fast switching speeds. Depending on the design of the field effect transistors, they can be used on the high side or the low side of many circuits in automobiles and other similar applications. More and more manufacturers are producing smart field effect transistors that are current limited and thermally protected. These various types of smart field effect transistors are designed to protect the power switch in case of a short circuit so that it will not overheat and over dissipate.
One example of a field effect transistor or other types of power device that is protected against destructive current surges, such as an electrical short, include the circuit disclosed in U.S. Pat. No. 4,355,344 to Felici et al. Ancillary transistors are formed in a semiconductor body and act as a parasitic transistor. In this complicated circuit, an external circuit includes a driver stage (DR) and a comparator (COMP) having as one input a reference voltage. The magnitude of the reference voltage is chosen, such that the comparator emits an inhibit signal only when the transistor T2 has undergone excessive heating, such as when the detected emitter voltage V.sub.E rises to a value indicating excessive heating of transistor T2. This signal then causes the driver stage DR to reduce the conduction of transistor T2 to a safe level.
U.S. Pat. No. 4,903,106 to Fukunaga et al. discloses another power device having a temperature detecting device formed on the same substrate to form a monolithic device. Two reference voltages are applied from a source to the base terminal of the bipolar transistor and through an error amplifier and driver to the gate of a field effect transistor. The reference voltage is applied to the base terminal of the transistor. Each collector terminal is connected to a power supply V.sub.CC through a resistor. The base-to-emitter voltage drop is lowered as the temperature is increased. A voltage value corresponding to an abnormal detecting temperature of the power device is supplied to the base terminal as a reference voltage because of the temperature dependency of the transistor.
Other prior art circuits include a bandgap reference that is formed on a semiconductor substrate, together with a field effect transistor and a parasitic bipolar transistor to form one monolithic device. The parasitic transistor and field effect transistor are manufactured by techniques known to those skilled in the art. However, the bandgap reference that is formed monolithically on one chip with other components adds to the expense and complexity of manufacturing the power devices, even though this smart field effect transistor circuit is simple in design with a parasitic bipolar transistor that turns on and initiates some type of thermal limiting.